Hello pmg2,
You have issues selecting an Express Transit card on your iPhone 12 Pro Max. We'll do our best to assist with this.
It sounds like you are currently up to date. We'd next make sure that you are within a supported region for transit cards: Where you can ride transit using Apple Pay - Apple Support As this feature isn't available in all regions that support the Wallet app.
With that said, this covers the exact steps you wish to take: Use Express Mode with transit cards, passes, and keys in Apple Wallet - Apple Support
Select Express Mode for a transit or payment card, pass, or key
When you add an eligible transit card or other compatible card, pass, or key to Apple Wallet, Express Mode is turned on by default. You can also change which card, pass, or key is set to Express Mode.
- Multiple cards, passes, and keys with Express Mode can be active in the Wallet app at the same time. For example, you can have one student ID and one car key each set to Express Mode.
- With some cards, passes, and keys, you can't have multiple of the same type or from the same issuer set to Express Mode. For example, you can only have one student ID from the same school set to Express Mode.
- With some passes, you might be able to set multiple passes of that type to Express Mode.
- For transit, you can set one transit card to Express Mode for each transit network. You can also set a payment card that can be used to pay your fare in locations where your Express Transit card isn't accepted.
How to select Express Mode for a transit or payment card, pass, or key on your iPhone
1. Open the Wallet app and select a card, pass, or key.
2. Tap the More button 
, then tap Express Transit Settings or Express Mode.
3. Select your preferred transit card, debit or credit card, or other compatible pass or key.
You can select a different transit or payment card for Express Transit Mode on your Apple Watch:
On your iPhone, open the Apple Watch app. In the My Watch tab, tap Wallet & Apple Pay. Then tap Express Transit Card, select a transit or payment card, and authenticate on your Apple Watch with your passcode.
If this continues to occur, it may be related to the specific card you are attempting to select or another reason. We'd suggest contacting Apple Support for their next steps: Apple Support
Wishing you well, cheers!
